A magnetic disturbance rippled around the Arctic Circle during the waning hours of March 1st, sparking bright auroras just as night fell over northern Europe. "The show was stunning and amazing," says Thomas Albin, who sends this picture from Esrange, Sweden.

A medium-speed solar wind stream is buffeting Earth's magnetic field. By itself, this was not enough to explain what happened. The extra ingredient was the IMF: the interplanetary magnetic field near Earth tilted south, opening a crack in our planet's magnetosphere. Solar wind poured in and fueled the display.

NOAA forecasters estimate a 15% to 25% chance of polar geomagnetic storms during the next 24 hours. High-latitude sky watchers should remain alert for auroras. www.spaceweather.com